Typically, a coil-energization magnetizing apparatus is used for the purpose of multipole magnetization of a ring-shaped permanent magnet rotor, which is incorporated in, for example, a radial-gap-type permanent-magnet-stepping-motor. This kind of magnetizing apparatus is, for example, structured as follows: a magnetic yoke is provided with a magnetization-object-containing hole into and from which a ring-shaped permanent magnet—which is a magnetization object—can be inserted and taken out; a number of grooves extending in the axial direction are formed in the inner wall surface of the magnetization-object-containing hole; insulating-coating wires are laid in the grooves, respectively, and the neighboring insulating-coating wires are connected to form a continuous zigzag coil. In this magnetizing apparatus, magnetization is carried out as follows. A magnetization object is set in the magnetization-object-containing hole, an electric charge stored in a capacitor is instantaneously discharged, and thereby a pulse current passes through the coil to generate a magnetic field, which magnetizes the magnetization object.
Meanwhile, as is commonly known, recent remarkable miniaturization of electronic devices has promoted reduction in the size and diameter of stepping motors used in such electronic devices. In conducting multipole magnetization of a ring-shaped permanent magnet to be used as a rotor, a large pulse current is applied using the coil-energization magnetizing apparatus of the type mentioned above. However, due to the reduction in diameter of the ring-shaped permanent magnet, the magnetization pitch (interval between magnetization poles) has become narrow, which in turn has resulted in reduction of the wire diameter of the coil provided in the magnetizing apparatus. As a result, only a limited amount of electric current can be supplied, leading to a problem that it is difficult to obtain sufficient magnetization characteristics.
As an approach to solve this problem, there is proposed a method for conducting multipole magnetization using at least four poles, the multipole magnetization being achieved by disposing a plurality of permanent magnets radially and thereby forming a plurality of alternative magnetic poles in the central portion, and placing a magnetization object in the central portion (refer to Patent Document 1). With this magnetizing apparatus utilizing permanent magnets, insufficient magnetization, which becomes problematic in cases where the pitch between the magnetic poles for magnetizing a magnetization object is narrowed down, can indeed be improved to a certain degree.
Still, the demand for further miniaturization (diameter reduction) and performance enhancement of stepping motors has been ever increasing in recent years. For example, for automatic focus mechanisms of portable imaging devices, stepping motors using ring-shaped permanent magnets which are magnetized so as to have multiple poles with narrow pitches, and which thereby allow high-precision control of lens actuators, have become important electronic components in order to achieve high-resolution images. For the ring-shaped permanent magnet constituting the rotor for this purpose, there are some requirements to be satisfied: achieving narrow pitch configuration—e.g., with a diameter of equal to or less than 3 mm and with ten or more magnetization poles—and at the same time, obtaining the magnetization characteristic of the saturation magnetization level. Conventional magnetizing methods including the above-mentioned magnetizing method utilizing permanent magnets are not effective enough to achieve such a magnetized structure; the required magnetization level cannot be met, and variation between the peak values of surface magnetic flux density may lead to a big problem.
As a technology aiming to improve insufficient magnetization, there is proposed a method of magnetizing a magnetization object utilizing the phenomenon that the magnetizing force required for reaching saturation is reduced in a hot atmosphere or liquid (for example, refer to Patent Document 2). The disclosure of Patent Document 2 is as follows. For example, for a Pr—Fe—B magnet, which is a kind of rare-earth permanent magnet, the required magnetizing force at 100 degrees centigrade is less than that at 25 degrees centigrade. Therefore, magnetization in this temperature range allows stable saturation magnetization to be achieved with a weak magnetic force.
However, actual magnetization using this method has revealed that, in preparation of a ring-shaped permanent magnet having a narrow magnetization pitch with multiple poles magnetized on an extremely small diameter as mentioned above, although the average surface-magnetic-flux-density peak value of all poles is improved to a certain degree, variation between the surface-magnetic-flux-density peak values is still significant, and therefore it is considerably difficult to obtain high magnetization quality.
Patent Document 1: Japanese Patent Application Laid-open Publication No. 2001-268860
Patent Document 2: Japanese Patent Application Laid-open Publication No. 6-140248